1. Field of the Invention
The present invention relates to a hydraulic clamp for fixedly securing an object such as a die, a workpiece and the like onto a machine tool such as an injection molding machine, a press machine and the like and more particularly to a wedge type multiplicatively forceful hydraulic clamp adapted to fixedly secure such an object strongly by the multiplication action of a wedge at the time of clamping.
2. Description of the Prior Art
Hitherto, as a wedge type multiplicatively forcefull hydraulic clamp there has been known the prior art proposed by the present inventor and disclosed in U.S. Pat. No. 4,504,046. The basic constitution of the prior art will be explained with reference to FIG. 14 as follows.
In a clamp housing 101 a drive chamber 103 is formed with its front end open. Into this drive chamber 103 a clamp arm 104 is inserted to be vertically pivotable therein. A fulcrum portion 104a is formed at the rear end of the bottom of the clamp arm 104, and a driven portion 104b is provided in the middle part of the top of the clamp arm 104. A clamping portion 104c is provided in the underside of the front part of the clamp arm 104. An upper portion of the drive chamber 103 is defined by a stationary wedge retaining surface 103a. A driving wedge 117 is inserted between the wedge retaining surface 103a and the driven portion 104b from behind. A hydraulic cylinder 105 is provided behind the driving wedge 117 so as to extend forward.
The driving wedge 117 is adapted to be moved forward and backward by the hydraulic cylinder 105. In the extended condition of the hydraulic cylinder 105 as indicated by a two-dot chain line, the driving wedge 117 is advanced so as to push down the clamp arm 104 through the driven portion 104b to the clamping position A. In the retracted condition of the hydraulic cylinder 105 as indicated by a solid line, the driving wedge 117 is retracted so as to release the driven portion 104b to the unclamping position B.
In the above basic constitution, as shown in FIG. 14, conventionally the driving wedge 117 is merely inserted directly between the wedge retaining surface 103a and the driven portion 104b.
According to the hydraulic clamp having such a basic constitution, a dynamical friction is prevailing between both the sliding contact surfaces of the driving wedge 117 and of the wedge retaining surface 103a and/or the driven portion 104b at the end of the clamping actuation and a statical friction is prevailing therebetween at the beginning of the unclamping actuation. Therefore, the hydraulic cylinder 105 is required to exert a larger retraction force at the beginning of the unclamping actuation than an extension force at the end of the clamping actuation correspondingly by the difference between the statical and dynamical frictional forces. As a result, there are following problems associated with the conventional constitution.
(a) A clamping force to be exerted by the hydraulic clamp is small.
Since the driving wedge 117 slides in direct contact with the wedge retaining surface 103a or the driven portion 104b, a coefficient of friction between both the sliding surface thereof is large.
In the case of the settlement of the unclamping force at a certain value, since the difference between the statical and dynamical frictional forces becomes larger by such a degree that the coefficient of friction between both the sliding contact surfaces is large, the clamping force becomes smaller correspondingly.
(b) A durability of the hydraulic clamp is low.
As the cumulate number of engagement and disengagement of the clamping wedge 117 is increased largely while the hydraulic clamp is used a long time, the sliding contact surfaces get damaged worse gradually. When the statical friction force between both the sliding contact surfaces becomes larger than a certain value, finally it becomes impossible to retract the driving wedge 117. Therefore, disadvantageously the service time of the hydraulic clamp becomes short.